This invention was made with Government support under Grant No. GM 37197 awarded by the National Institutes of Health. The Government has certain rights in this invention.
1. Field of the Invention
This invention pertains generally to obtaining a visual image of an object using magnetic resonance imaging, and more particularly to a method for constructing, from axial anisotropic diffusion weighted images of the same imaging slice, a color image in which the spectral frequency of each pixel in the image reflects the direction of anisotropic water motion in the object.
2. Description of the Background Art
Magnetic resonance imaging (MRI) has proven to be one of the most versatile non-invasive imaging techniques in biomedicine capable of providing anatomical as well as functional information. Using appropriate motion encoding techniques, MRI can also provide images exhibiting quantitative information regarding motion of water molecules. While MRI dealing with macroscopic motion is generally referred to as flow imaging, MRI dealing with microscopic motion within a single voxel is referred to as diffusion weighted imaging.
The original Stejskal-Tanner sequence for measurement of molecular diffusion by nuclear magnetic resonance (NMR) employed diffusion gradient pulses (originally called time-dependent field gradient) to encode quantitative information regarding molecular motion (diffusion coefficient) into a signal intensity. The MRI sequence for diffusion weighted imaging (DWI) is an extension of this original Stejskal-Tanner sequence where differences in intravoxel water motion (a factor often referred to as apparent diffusion coefficient) are treated as another contrast mechanism.
Intravoxel anisotropic water motion produces signal attenuation of diffusion weighted images (DWIs) dependent on the direction of the diffusion gradient pulses applied. Such directional dependency, which is most conspicuous in the myelinated fibers of the nervous system, is thought to be due to anisotropic restriction of water diffusion. Therefore, DWIs obtained using diffusion gradient pulses applied to only one spatial axis are generally referred to as anisotropic DWIs, in contrast to conventional DWIs where diffusion gradient pulses are applied to three axial directions simultaneously.
Anisotropic DWIs contain information regarding intravoxel water motion in space projected onto the corresponding axis. While intravoxel isotropic water motion produces identical effects on signal intensity of anisotropic DWIs regardless of the direction of the diffusion gradient pulses, intravoxel anisotropic water motion affects signal intensity in a manner dependent on the angle of the direction of anisotropy with respect to the spatial axis in which the diffusion gradient pulses have been applied. Therefore, each anisotropic DWI can be considered the projection image of intravoxel anisotropic water motion. However, each pixel of anisotropic DWIs, the spatial information contained in which has three dimensional resolution, carries only one dimensional information regarding anisotropic water motion in space. As a result, image resolution is not as great as that where each pixel contains three dimensional information.
Therefore, there is a need for a method of reconstructing three dimensional information regarding intravoxel water motion from axial anisotropic DWI projection images of the identical imaging plane. The present invention satisfies that need, as well as others, and overcomes the deficiencies in conventional imaging methods.